1. Field of the Invention
The present invention relates to a switching power supply apparatus that includes a transformer and two switching devices.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2001-37220 discloses a switching power supply apparatus having a configuration in which two switching devices are alternately switched on/off in a complementary manner. The switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2001-37220 includes a flyback converter and two switching devices, i.e., high-side and low-side switching devices (FETs). During an ON period of the low-side FET, energy is stored in a transformer, and the energy stored in the transformer is supplied to a load during an OFF period of the low-side FET. The switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2001-37220 uses an active clamp method in which a surge voltage applied to the low-side FET is clamped, and a zero-voltage switching operation of the two switching devices is achieved.
In this switching power supply apparatus, the low-side FET, the primary winding of the transformer, and a DC power supply are connected in series with one another, and a series circuit including the high-side FET and a capacitor is connected between the two ends of the primary winding of the transformer. The transformer of the switching power supply apparatus includes first and second drive windings, and one end of the first drive winding is connected to a low-side switching control circuit. The low-side switching control circuit outputs a control signal to the gate of the low-side FET. The source of the high-side FET is connected to the drain of the low-side FET. A high-side switching control circuit includes a time constant circuit including a capacitor and a resistor.
The switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2001-37220 is configured such that the high-side FET is driven by the second drive winding of the transformer and the time constant circuit. However, when a PFC converter is provided or a DC-DC converter which also functions as a PFC converter is provided, the low-side FET needs to be switched on/off in accordance with variations in the input voltage so as to improve a power factor. In this case, in the switching power supply apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2001-37220, since an ON period of the high-side FET is substantially fixed by the time constant circuit, it is difficult to enable the changing of the ON period of the high-side FET. On the other hand, in order to efficiently drive the FETs, the high-side FET may be controlled in accordance with ON/OFF operations of the low-side FET by including a driver IC in the high-side switching control circuit. However, a reduction in cost is difficult to achieve in this case.